1.1 Field of the Invention
The present invention concerns wireless communications. More specifically, the present invention concerns improving wireless communications, such as communications taking place in a wireless LAN.
1.2. Background Information
Recent advances in the areas of wireless communications, digital signal processing, and VLSI make it feasible to provide wireless networking with high capacity and coverage at reasonably low cost. In the past few years, various wireless networking standards (e.g., IEEE 802.11, IEEE 802.15, etc.) have been ratified and corresponding products deployed in the field. Medium access control protocols for these standards control access to the transmission medium and provide for an orderly and efficient use of that capacity. A popular wireless protocol—IEEE 802.11—is introduced below.
1.2.1 IEEE 802.11 Medium Access Control (MAC) Protocol
IEEE 802.11 was the first international standard for indoor wireless local-area networks (WLAN). It provides physical layer rates of 1 and 2 Mbps. IEEE 802.11b was introduced later in 1999. It uses three (3) different modulation schemes (i.e., Differential Binary Phase Shift Keying (DBPSK), Differential Quaternary Phase Shift Keying (DQPSK), and Complementary Code Keying (CCK)) which provide a total of four different physical layer rates, ranging from 1 to 11 Mbps.
The basic MAC protocol of IEEE 802.11b is the distributed coordination function (DCF) which employs carrier sense multiple access with collision avoidance (CSMA/CA). It is contention-based and uses both physical carrier sensing and virtual carrier sensing to avoid collisions. When a station (STA) has data to send, it first senses the media to check if other STAs are transmitting. (Note that the use of “station” in this application is not intended to imply that a node or terminal is stationary. Thus, “station” may include any type of node, including mobile devices.) If other STAs are not transmitting, a request to send (RTS) packet is sent and the intended receiver will reply with a clear to send (CTS) packet. These two control packets are used to set a network allocation vector (NAV), in which the channel reservation information is stored, for all the stations to avoid the hidden terminal problem.
After successfully exchanging the control packets, a data packet will be sent and the destination station will send back an acknowledgment (ACK) if the packet has been received without error.
IEEE 802.11 MAC protocol supports multirate capability. Each station can select an appropriate rate to transmit the data, based upon the perceived ambient channel condition. The rate selection/adaptation algorithm is left as implementation dependent in the IEEE 802.11 standard. However, all rate adaptation algorithms follow a common principle outlined in the '773 provisional and described below.
Assume all stations are equipped with a single transmitting and receiving antenna. The available bandwidth is W Hz (11 MHz for 802.11b). Transmission power for each station is P0 Watts. If the distance between the transmitter and receiver is d meters, the received signal power would be:P=P0d−η  Equation 1where η is the path loss exponent (PLE) and usually varies between 2 and 4.
The received signal is also subject to fading due to the multipath effect. In a typical indoor environment, where there is no line-of-sight transmission, the Rayleigh fading model is an appropriate model. Its probability density function (pdf) is given by:
                              p          ⁡                      (            r            )                          =                              r                          σ              2                                ⁢                      exp            ⁡                          (                              -                                  r                                      2                    ⁢                                                                                  ⁢                                          σ                      2                                                                                  )                                                          Equation        ⁢                                  ⁢        2            where σ2 is the time-average power of the received signal before envelope detection and r is the distance between the transmitter and receiver.
In the IEEE 802.11b, the Direct Sequence Spread Spectrum (DSSS) physical layer operates in the 2.4 GHz ISM band, and three different modulation schemes are used. They are: Differential Binary Phase Shift Keying (DBPSK) for the 1 Mbps data rate, Differential Quaternary Phase Shift Keying (DQPSK) for the 2 Mbps data rate, and Complementary Code Keying (CCK) for the 5.5 Mbps and 11 Mbps data rates, respectively. The control packets and header part of the data packets are always modulated using DBPSK. The modulation scheme of the data frame is indicated in the physical layer header of the data packet.
For DBPSK modulation the bit error probability in the AWGN channel is:
                              p          e                =                              1            2                    ⁢                      exp            ⁡                          (                              -                                                      E                    b                                                        N                    0                                                              )                                                          Equation        ⁢                                  ⁢        3            where Eb is the per bit energy of the transmitted signal and N0 is the power spectral density of the additive white Gaussian noise.
For DQPSK modulation, the error probability is
                              p          e                =                                            Q              1                        ⁡                          (                              a                ,                b                            )                                -                                    1              2                        ⁢                                          I                0                            ⁡                              (                                  a                  ,                  b                                )                                      ⁢                          exp              ⁡                              (                                                      -                                          1                      2                                                        ⁢                                      (                                                                  a                        2                                            +                                              b                        2                                                              )                                                  )                                                                        Equation        ⁢                                  ⁢        4            where Q1(a,b) is the Marcum Q function and I0(a,b) is the modified zero order Bessel function. a and b are defined as:
                    {                                                            a                =                                                                                                    2                        ⁢                                                  E                          b                                                                                            N                        0                                                              ⁢                                          (                                              1                        -                                                                              1                            2                                                                                              )                                                                                                                                              b                =                                                                                                    2                        ⁢                                                  E                          b                                                                                            N                        0                                                              ⁢                                          (                                              1                        +                                                                              1                            2                                                                                              )                                                                                                                              Equation        ⁢                                  ⁢        5            
For DBPSK modulation the system processing gain is 11 and 5.5 for DQPSK modulation. The 5.5 and 11 Mbps data rate use CCK as the modulation scheme and their processing gain is 2 and 1 respectively. The bit error probability is:
                                          P            e                    =                      1            -                                          ∫                                  -                  X                                ∞                            ⁢                                                                    (                                                                  1                                                                              2                            ⁢                                                                                                                  ⁢                            π                                                                                              ⁢                                                                        ∫                                                                                    -                              v                                                        +                            X                                                                                v                            +                            X                                                                          ⁢                                                                              exp                            ⁡                                                          (                                                              -                                                                                                      y                                    2                                                                    2                                                                                            )                                                                                ⁢                                                                                                          ⁢                                                      ⅆ                            y                                                                                                                )                                                                              M                      2                                        -                    1                                                  ⁢                                  exp                  ⁡                                      (                                          -                                                                        v                          2                                                2                                                              )                                                  ⁢                                                                  ⁢                                  ⅆ                  v                                                                    ⁢                                  ⁢        Where                            Equation        ⁢                                  ⁢        6                                X        =                                            E              b                                      N              0                                                          Equation        ⁢                                  ⁢        7            
The performance curves of bit error rate (BER) versus signal-to-noise ratio (SNR) for different modulation scheme are shown in FIG. 1 of the '773 provisional. Using the above bit error probability and assume independent random error for each bit, the packet error rate (PER) for a L-octet data packet can be calculated as:P=1−(1−Pe)8L  Equation 8
If the quality-of-service parameters (bit error rate (BER), packet error rate (PER), etc.) are given, the most suitable modulation scheme and the corresponding transmission rate can always be found based on the distance between the transmitter and receiver using the equations given above. In a real implementation, each station may estimate the ambient channel condition by measuring the received signal power strength and then determine which transmission rate can yield the best BER/SNR performance.
Various amendments (e.g., IEEE 802.11e, IEEE 802.11g, etc) to IEEE 802.11 have been released to improve or enhance a certain aspect of the legacy standard. Nevertheless, no change has been made, with respect to the direct data transmission approach from the transmitter to the receiver.
1.2.1.1 Perceived Limitations of the Current IEEE 802.11
Since the MAC scheme is contention-based and the transmission rate of different stations can vary widely (e.g., from 1 to 11 Mbps in IEEE 802.11b), if all the stations have uniform traffic to/from the access point (AP), the low data rate stations will use much more channel time than the high data rate stations. This results in a significant degradation of network throughput as well as average delay perceived by all transmitting stations.
Moreover, this MAC scheme suffers from serious fairness problems. More specifically, the high rate stations have the same channel access probability as the low rate stations, but in fact they obtain a lower share of channel occupation time than the low rate stations.
In view of the foregoing limitations of the current IEEE 802.11 protocol, a new MAC protocol that can achieve better performance and provide fair service would be useful. It would also be useful if such an approach could also reduce interference and improve coverage in an area covered by multiple access points. It would be useful if such an approach were backwards compatible with current IEEE 802.11 standards. It would be useful if such an approach could be used with other wireless techniques and protocols.